As of today there is not a raw Silverblue image to flash on an SD card, despite the fact that Silverblue is available for aarch64 architectures.
In order to install Fedora Silverblue on the Raspberry Pi, there are various guides where you need to prepare the SD card adding UEFI and some other tricks.
So, why not install Fedora IoT, then rebase to Fedora Silverblue? The power of ostree.